Double-resonance mri coil

ABSTRACT

A double-resonance coil for use in NMR spectroscopy and imaging having a coil element, a first input/output terminal coupled to the coil element, and a second input/output terminal coupled to the coil element. A first resonance means is coupled between the coil element and the first input/output terminal to cause the NMR coil to resonate at a first NMR frequency, and a second resonance means is coupled between the coil element and the second input/output terminal to cause the NMR coil to resonate at a second NMR frequency substantially different than the first NMR frequency. The double resonance coil includes a first frequency-blocking means coupled to the coil element for substantially preventing the second NMR frequency from being detected at the first input/output terminal and a second frequency-blocking means coupled to the coil element for substantially preventing the first NMR frequency from being detected at the second input/output terminal.

[0001] This is a continuation-in-part of U.S. Ser. No. 08/071,582, entitled “Oxygen-17 NMR Spectroscopy and Imaging in the Human,” filed Jun. 2, 1993.

BACKGROUND OF THE INVENTION

[0002] The present invention relates to a double-resonance coil for use in NMR spectroscopy and imaging which allows simultaneous radiation and detection at two different radio frequencies.

[0003] Nuclear magnetic resonance (NMR) spectroscopy and imaging are used in various medical applications. For example, conventional NMR imaging systems in widespread use generate an anatomical image of a selected portion of the human body by utilizing a radio-frequency (RF) coil to irradiate the selected body portion with radiation at a frequency of 63.9 MHz, which causes nuclear magnetic resonance of hydrogen-1 constituents (protons) in the selected body portion at 1.5 Tesla. The image is then generated based on changes in the nuclear signal sensed via the RF coil.

[0004] It has been suggested that dual-frequency NMR imaging, which would generate and detect RF energy at two frequencies, could be utilized via a double-resonance NMR coil. For example, in an abstract entitled Oxygen-17: A Physiological, Biochemical and Anatomical MRI Contrast Agent from the Seventh Annual Meeting of the Society of Magnetic Resonance in Medicine, Mateescu, et al. state: “A ¹⁷O/¹H double resonance probe was used in experiments in which reliable superposition of oxygen and proton images was sought.”

[0005] In U.S. Pat. No. 4,742,304, Schnall, et al. disclose a multiple-tuned NMR probe which can radiate RF energy and detect changes in the RF energy at a number of different frequencies. While the patent states that the NMR probe allows “simultaneous study of different nuclei” (col. 1, lines 7-8), it is apparent that the NMR probe is excited with the different NMR frequencies consecutively, not simultaneously. The Schnall, et al. probe's inability to be used in simultaneous NMR spectroscopy and/or imaging is apparent from the fact it only has one input/output terminal: “It is another object of the present invention to provide such a multiple-tuning NMR probe in which all frequencies are generated and detected using a single input/output terminal.” Column 2, lines 26-29.

SUMMARY OF THE INVENTION

[0006] The present invention is directed to a double-resonance coil for use in NMR spectroscopy and imaging. The double-resonance coil includes a coil element, a first input/output terminal coupled to the coil element, and a second input/output terminal coupled to the coil element. A first resonance means is coupled between the coil element and the first input/output terminal to cause the double-resonance coil to resonate at a first NMR frequency, and a second resonance means is coupled between the coil element and the second input/output terminal to cause the double-resonance coil to resonate at a second NMR frequency different than the first NMR frequency. The double-resonance coil includes a first frequency-blocking means coupled to the coil element for substantially preventing the second NMR frequency from being detected at the first input/output terminal and a second frequency-blocking means coupled to the coil element for substantially preventing the first NMR frequency from being detected at the second input/output terminal.

[0007] The double-resonance coil in accordance with the present invention would allow, for example, simultaneous excitation of oxygen-17 and hydrogen-1 isotopes present in a portion of a human patient in order to provide accurate superposition of anatomical images produced by hydrogen-1 resonance with images relating to blood flow or oxygen consumption produced by oxygen-17 resonance. Alternatively, the double-resonance coil could be used to provide simultaneous excitation of xenon-129 and hydrogen-1 constituents in order to, for example, provide accurate superposition of anatomical images produced by hydrogen-1 resonance with images relating to blood flow produced by xenon-129 resonance. The double-resonance coil could also be used to provide simultaneous excitation of xenon-129 and oxygen-17 constituents in order to, for example, provide accurate superposition of images produced by xenon-129 resonance with images produced by oxygen-17 resonance.

[0008] The features and advantages of the present invention will be apparent to those of ordinary skill in the art in view of the detailed description of the preferred embodiment, which is made with reference to the drawings, a brief description of which is provided below.

BRIEF DESCRIPTION OF THE DRAWINGS

[0009]FIG. 1 illustrates an NMR spectroscopy and imaging system for use in connection with humans; and

[0010]FIG. 2 is a diagram of an embodiment of the RF coil of FIG. 1.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

[0011]FIG. 1 illustrates a nuclear magnetic resonance (NMR) spectroscopy and imaging system 10 which operates by magnetically resonating NMR constituents or isotopes such as hydrogen-1 or oxygen-17 in a portion of a human patient. The NMR system 10 includes a plurality of elements for generating various magnetic fields to produce magnetic resonance of the NMR constituents. These elements include a whole body, superconducting magnet 12 for generating a uniform, static magnetic field. The magnet 12 is referred to as a “whole body” magnet since its circumference is large enough to accommodate the entire body controlled via a controllable power supply 20; a number of gradient field amplifiers 24 controls the fields generated by the gradient field coils 14 via three lines 26, one line for each of the x, y, z directions. A pair of transceivers 30, 31 and a pair of associated modulators 32, 33 control the field generated by the RF coil 16. The transceivers 30, 31 also detect and receive electrical NMR signals generated from the magnetic resonance of the NMR constituents and supply those signals to a conventional I/O interface 36 of a controller 40. The controllable power supply 20 and the gradient field amplifiers 24 are also connected to the I/O interface 36 via lines 42, 44, respectively.

[0012] The I/O interface 36 includes two pairs of analog-to-digital (A/D) converters (not shown), each of which is connected to receive an input after quadrature detection from a respective one of the transceivers 30, 31.

[0013] The controller 40 also includes a processor 50, a random-access memory (RAM) 52, and a read-only memory (ROM) 54, all of which are interconnected via a data bus 56. The processor 50 may process the data (NMR signals) received from the two transceivers 30, 31 on a time-shared basis. Alternatively, separate processors 50 could be provided to process the data from the two transceivers 30, 31 simultaneously. An image display device 58, such as a cathode ray tube (CRT), is also connected to the controller 40. Additional components, such as a boot and editing terminal 60, a tape drive 62 for storing data relating to images, and a satellite station 64 for performing spectroscopy may also be connected to the controller 40.

[0014] During operation, the NMR system 10 generates a uniform static magnetic field via the superconducting magnet 12 and shim coils 13 and selectively generates the gradient and resonating magnetic fields via the coils 14, 16 so as to generate NMR signals corresponding to a selected volume element of the human subject.

[0015] As described in U.S. Ser. No. 08/071,582, entitled “Oxygen-17 NMR Spectroscopy and Imaging in the Human,” filed Jun. 2, 1993 and assigned to the same assignee, the NMR system 10 could be used to generate oxygen-17 images representing the magnitude of blood flow and/or the rate of oxygen consumption in a human.

[0016] An RF coil 16 for allowing either simultaneous or consecutive magnetic resonance of NMR constituents in a human body portion via RF excitation at two different frequencies is shown in FIG. 2. The RF coil 16 could be used to provide simultaneous excitation of oxygen-17 and hydrogen-1 constituents in the human body portion in order to, for example, provide accurate superposition of anatomical images produced by hydrogen-1 resonance with images relating to blood flow or oxygen consumption produced by oxygen-17 resonance. Alternatively, the RF coil 16 could be used to provide simultaneous (or consecutive) excitation of xenon-129 and hydrogen-1 constituents in order to, for example, provide accurate superposition of anatomical images produced by hydrogen-1 resonance with images relating to blood flow produced by xenon-129 resonance. The double-resonance coil could also be used to provide simultaneous excitation of xenon-129 and oxygen-17 constituents in order to, for example, provide accurate superposition of images produced by xenon-129 resonance with images produced by oxygen-17 resonance.

[0017] As used herein, “simultaneous excitation” means that the RF coil 16 is provided with two different RF frequencies, one RF frequency at each of its two input/output terminals, at the same instant in time. The term “consecutive excitation” means that two RP frequencies provided to the two input/output terminals are alternated in time so that neither RF frequency is provided at the same instant in time as the other RP frequency.

[0018] Referring to FIG. 2, the RF coil 16 includes a circular coil element 100 formed of a single loop of copper tubing and is used about the head of the patient. The diameter of the coil element 100 is 11 centimeters and the diameter of the copper tubing is 0.5 cm. The RF coil 16 has a first input/output terminal 102 connected to the left side of the coil element 100 as shown in FIG. 2 and a second input/output terminal 104 connected to the right side of the coil element 100. The coil element 100 has a first end point 106 on its left side at which point the curved portion of the coil element 100 ends and a second end point 108 on its right side at which the curved portion of the coil element 100 ends.

[0019] A number of circuit components are connected between the end point 106 of the coil element 100 and the input/output terminal 102 of the coil element 100. Those circuit elements include two coaxial cables 110 a, 110 b, a pair of capacitors 111, 112, a variable tuning capacitor 113, a variable impedance-matching capacitor 114, and a pair of capacitors 116, 118 connected in parallel with the impedance-matching capacitor 114. The tuning capacitor 113 is used to tune the circuit to resonate at the RF frequency associated with the input/output 102 terminal. The impedance-matching capacitor 114 is used for conventional impedance-matching purposes. The length of each of the coaxial cables 110 a, 110 b (which are open at their lower ends) is substantially one-fourth of the wavelength of the RF signal associated with the input/output terminal 104 so that the cables 110 act as a very low impedance (theoretically zero impedance) to prevent the RF signal provided at the terminal 104 from being detected at the terminal 102. One cable 110 may be used, although two λ/4 coaxial cables 110 are preferred to obtain better isolation.

[0020] A number of circuit components are connected between the end point 108 of the coil element 100 and the input/output terminal 104 of the coil element 100. Those circuit elements include a smaller coil element 120, a variable tuning capacitor 122, a variable impedance-matching capacitor 124, and three capacitors 126, 128 and 130 connected to ground. The tuning capacitor 122 is used to fine tune the circuit to resonate at the RF frequency associated with the input/output terminal 104. The impedance-matching capacitor 124 is used for conventional impedance-matching purposes.

[0021] The smaller coil element 120 is formed of two circular loops, each of which is three centimeters in diameter, of the same copper tubing of the coil element 100, so that the coil element 120 generates a magnetic field in the same direction as the static field generated by the superconducting magnet 12. The smaller coil element 120 and the main coil element 100 are made as one rigid piece. The inductance of the smaller coil element 120 is 50 ohms, which matches the impedance of a coaxial cable.

[0022] The purpose of the smaller coil element 120 is to prevent the RF signal provided at the input/output terminal 102 from being detected at the input/output terminal 104 by acting as a low impedance to the RF signal provided at the input/output terminal 102. The impedance of the smaller coil element 120 (which is primarily inductance) is equal to 2πfL, where f is the frequency of the signal provided to the coil element 120 and L is the inductance of the coil element 120. It can be seen that for lower frequencies, the impedance of the coil element 120 is smaller. Thus, when a relatively low frequency signal is provided to the input/output terminal 102 and a relatively high frequency signal is provided to the terminal 104, the low impedance of the smaller coil element 120 substantially prevents the relatively low frequency signal from being detected at the terminal 104.

[0023] A first resonant frequency f₁ of the NMR coil 10, as seen at the left input/output terminal 102, is determined by the total capacitance C₁ of the components in the left branch of the NMR coil between the point 108 and the terminal 102 and the inductance L of the coil element 100 in accordance with the equation f₁=1/(2π{square root}{square root over (LC)}₁). Similarly, a second resonant frequency f₂ of the NMR coil 10, as seen at the right input/output terminal 104, is determined by the total capacitance C₂ of the components in the right branch of the NMR coil between the point 106 and the terminal 104 and the inductance L of the coil element 100 in accordance with the equation f₂=1/(2π{square root}{square root over (LC)}₂).

[0024] In operation, the left input/output terminal 102 of the RF coil 16 is connected to receive and transmit RF signals at a first NMR frequency, and the right input/output terminal 104 is connected to receive and transmit RF signals at a second NMR frequency. The first NMR frequency should be lower than the second NMR frequency.

[0025] The NMR frequency necessary to cause magnetic resonance of an NMR constituent or isotope present in a human body portion is defined in accordance with the well-known Larmor equation set forth below:

f=Hγ12π

[0026] where H is the magnitude of the static magnetic field, where γ is the gyromagnetic ratio of the isotope to be magnetically resonated, and where f is the NMR frequency necessary to cause nuclear magnetic resonance. The NMR frequency necessary to magnetically resonate oxygen-17 in a static magnetic field of 1.5 Tesla is 8.66 MHz; the NMR frequency necessary to magnetically resonate hydrogen-1 in a static magnetic field of 1.5 Tesla is 63.9 MHz; and the NMR frequency necessary to magnetically resonate xenon-129 in a static magnetic field of 1.5 Tesla is 17.6 MHz.

[0027] If the RF coil 16 were used to simultaneously generate images based on dual oxygen-17/hydrogen-1 resonance, the input/output terminal 102 would be connected to a transceiver operable at 8.66 MHz (the Larmor frequency for oxygen-17 at 1.5 Tesla), and the input/output terminal 104 would be connected to a transceiver operable at 63.9 MHz (the Larmor frequency for hydrogen-1 at 1.5 Tesla). The tuning capacitor 113 would be adjusted so that the coil element 100 resonates at 8.66 MHz when an RF signal of that frequency is provided to the input/output terminal 102, and the tuning capacitor 122 would be adjusted so that the coil element 100 resonates at 63.9 MHz when an RF signal of that frequency is provided to the input/output terminal 104. The length of the coaxial cables 110 would be selected to be about one-fourth of the hydrogen-1 wavelength (63.9 MHz)(79 cm for RG/U400 coaxial cable), and the capacitors in the circuit would have the values indicated in the table set forth below. NMR COIL FOR SIMULTANEOUS ¹⁷O/¹H RADIATION Capacitor Value 111 330 pF 112 330 pF 113 2-120 pF 114 1-18 pF 116 10 pF 118 10 pF 122 1-10 pF 124 1-10 pF 126 10 pF 128 10 pF 130 10 pF

[0028] If the RF coil 16 were used to simultaneously generate images based on dual xenon-129/hydrogen-1 resonance, the input/output terminal 102 would be connected to a transceiver operable at 17.6 MHz (the Larmor frequency for xenon-129 at 1.5 Tesla), and the input/output terminal 104 would be connected to a transceiver operable at 63.9 MHz (the Larmor frequency for hydrogen-1 at 1.5 Tesla). The tuning capacitor 113 would be adjusted so that the coil element 100 resonates at 17.6 MHz when an RF signal of that frequency is provided to the input/output terminal 102, and the tuning capacitor 122 would be adjusted so that the coil element 100 resonates at 63.9 MHz when an RF signal of that frequency is provided to the input/output terminal 104. The length of the coaxial cables 110 would be selected to be about one-fourth of the hydrogen-1 wavelength (63.9 MHz), and the capacitors in the circuit would have the values indicated in the table set forth below. NMR COIL FOR SIMULTANEOUS ¹²⁹Xe/¹H RADIATION Capacitor Value 111 80 pF 112 80 pF 113 2-120 pF 114 1-18 pF 116 2.4 pF 118 2.4 pF 122 1-10 pF 124 1-10 pF 126 10 pF 128 10 pF 130 10 pF

[0029] If the RF coil 16 were used to simultaneously generate images based on dual oxygen-17/xenon-129 resonance, the input/output terminal 102 would be connected to a transceiver operable at 8.66 MHz (the Larmor frequency for oxygen-17 at 1.5 Tesla), and the input/output terminal 104 would be connected to a transceiver operable at 17.6 MHz (the Larmor frequency for xenon-129 at 1.5 Tesla). The tuning capacitor 113 would be adjusted so that the coil element 100 resonates at 8.66 MHz when an RF signal of that frequency is provided to the input/output terminal 102, and the tuning capacitor 122 would be adjusted so that the coil element 100 resonates at 17.6 MHz when an RF signal of that frequency is provided to the input/output terminal 104. The length of the coaxial cables 110 would be selected to be about one-fourth of the xenon-129 wavelength (17.6 MHz), and the capacitors in the circuit would have the values indicated in the table set forth below. NMR COIL FOR SIMULTANEOUS ¹⁷O/¹²⁹Xe RADIATION Capacitor Value 111 330 pF 112 330 pF 113 2-120 pF 114 1-18 pF 116 10 pF 118 10 pF 122 1-10 pF 124 1-10 pF 126 131 pF 128 131 pF 130 131 pF

[0030] Although the hydrogen-1, oxygen-17, and xenon-129 NMR frequencies have been specifically discussed, other NMR frequencies could be utilized in connection with the double-resonant coil of the present invention.

[0031] Numerous modifications and alternative embodiments of the invention will be apparent to those skilled in the art in view of the foregoing description. This description is to be construed as illustrative only, and is for the purpose of teaching those skilled in the art the best mode of carrying out the invention. The details of the structure and method may be varied substantially without departing from the spirit of the invention, and the exclusive use of all modifications which come within the scope of the appended claims is reserved. 

What is claimed is:
 1. A double-resonance NMR coil for use in nuclear magnetic resonance, said double-resonance NMR coil comprising: a coil element; a first input/output terminal coupled to said coil element; a second input/output terminal coupled to said coil element; first resonance means coupled between said coil element and said first input/output terminal for causing said NMR coil to resonate at a first NMR frequency; second resonance means coupled between said coil element and said second input/output terminal for causing said NMR coil to resonate at a second NMR frequency, said second NMR frequency being different than said first NMR frequency; first frequency-blocking means coupled to said coil element for substantially preventing said second frequency from being detected at said first input/output terminal; and second frequency-blocking means coupled to said coil element for substantially preventing said first frequency from being detected at said second input/output terminal.
 2. An NMR coil as defined in claim 1 wherein said coil element comprises a coil with a single substantially circular loop.
 3. An NMR coil as defined in claim 1 wherein said first resonance means comprises a plurality of capacitors, one of said capacitors being a tuning capacitor.
 4. An NMR coil as defined in claim 3 wherein said second resonance means comprises a plurality of capacitors, one of said capacitors being a tuning capacitor.
 5. An NMR coil as defined in claim 1 wherein said first frequency blocking means comprises a coaxial cable having a length.
 6. An NMR coil as defined in claim 1 wherein said length of said coaxial cable is substantially equal to one-fourth of the wavelength associated with said second NMR frequency.
 7. An NMR coil as defined in claim 6 wherein said first frequency blocking means additionally comprises a second coaxial cable having a length substantially equal to one-fourth of the wavelength associated with said second NMR frequency.
 8. An NMR coil as defined in claim 1 wherein said second frequency-blocking means comprises a second coil element.
 9. An NMR coil as defined in claim 7 wherein said second coil element comprises a coil having two substantially circular loops. 